Resistance spot welding is a process used by a number of industries to join together two or more metal workpieces. The automotive industry, for example, often uses resistance spot welding to join together pre-fabricated sheet metal layers during the manufacture of a vehicle body panel for a door, hood, trunk lid, or lift gate, among others. A number of spot welds are typically formed along a peripheral edge of the sheet metal layers or some other bonding region to ensure the body panel is structurally sound. While spot welding has typically been practiced to join together certain similarly-composed metal workpieces—such as steel-to-steel and aluminum alloy-to-aluminum alloy—the desire to incorporate lighter weight materials into a vehicle platform has generated interest in joining steel workpieces to aluminum alloy workpieces by resistance spot welding.
Resistance spot welding, in general, relies on the resistance to the flow of an electrical current through contacting metal workpieces and across their faying interface (i.e., the contacting interface of the metal workpieces) to generate heat. To carry out such a welding process, a pair of opposed spot welding electrodes are typically clamped at diametrically aligned spots on opposite sides of the workpieces at a predetermined weld site. An electrical current is then passed through the metal workpieces from one electrode to the other. Resistance to the flow of this electrical current generates heat within the metal workpieces and at their faying interface. When the metal workpieces being welded are a steel workpiece and an aluminum alloy workpiece, the heat generated at the faying interface initiates a molten weld pool in the aluminum alloy workpiece. This molten aluminum alloy weld pool wets the adjacent surface of the steel workpiece and, upon stoppage of the current flow, solidifies into a weld joint. After the spot welding process has been completed, the welding electrodes are retracted from their respective workpiece surfaces, and the spot welding process is repeated at another weld site.
Spot welding a steel workpiece to an aluminum alloy workpiece presents some challenges. These two types of metals have several considerable dissimilarities that tend to disrupt the welding process. Specifically, steel has a relatively high melting point (˜1500° C.) and a relatively high resistivity, while the aluminum alloy has a relatively low melting point (˜600° C.) and a relatively low resistivity. As a result of these physical differences, the aluminum alloy melts more quickly and at a much lower temperature than steel during current flow. The aluminum alloy also cools down more quickly than steel after current flow has been terminated. Thus, immediately after the welding current stops, a situation occurs where heat is not disseminated symmetrically from the weld site but, rather, is conducted from the hotter steel workpiece through the aluminum alloy workpiece towards the electrode on the aluminum alloy side.
The development of a steep thermal gradient between the steel workpiece and the aluminum alloy-side welding electrode is believed to weaken the integrity of the resultant weld joint in two primary ways. First, because the steel workpiece retains heat for a longer duration than the aluminum alloy workpiece after the welding current has stopped, the molten aluminum alloy weld pool solidifies directionally, starting from the region nearest the colder welding electrode associated with the aluminum alloy workpiece and propagating towards the faying interface. A solidification path of this kind tends to force defects—such as gas porosity, shrinkage, micro-cracking, and oxide residue—towards and along the faying interface within the weld joint. Second, the sustained elevated temperature in the steel workpiece causes the growth of brittle Fe—Al intermetallic compounds at and along the faying interface. Having a dispersion of nugget defects together with excessive of Fe—Al intermetallic compounds along the faying interface tends to reduce the peel strength of the weld joint established between the workpieces.
Another notable dissimilarity between the two metals is that the aluminum alloy contains one or more refractory oxide layers (hereafter collectively referred to as “oxide layer”) on its surface that are created during mill operations (e.g., annealing, solution treatment, casting, etc.) and environmental exposure. This oxide layer, which is composed primarily of aluminum oxides, is electrically insulating, mechanically tough, and self-healing in air. Such characteristics are not conducive to the mechanics of spot welding a steel workpiece to an aluminum alloy workpiece. In particular, the surface oxide layer raises the electrical contact resistance of an aluminum alloy workpiece—namely, at its faying surface and at its electrode contact point—making it difficult to effectively control and concentrate heat within the aluminum alloy workpiece. The mechanical toughness of the surface oxide layer also hinders wetting of the steel workpiece. The problems posed by the refractory oxide layer on the surface of the aluminum alloy workpiece are further complicated by the fact that the oxide layer can self-heal or regenerate if breached in the presence of oxygen.
Furthermore, in order to obtain a reasonable weld bond area between a steel workpiece and an aluminum alloy workpiece, there is generally the need to employ a weld schedule that specifies higher currents, longer weld times, or both as compared to spot welding steel-to-steel, which can damage the welding electrodes. For example, if a zinc-coated steel workpiece is being spot welded to an aluminum alloy workpiece under these more aggressive weld schedules, the welding electrode in contact with the steel workpiece has a tendency to react with the zinc coating to form a layer of brass. Surface expulsion can also occur at the interface of the steel workpiece and the contacting welding electrode if the applied welding current is too high. For the welding electrode in contact with the aluminum alloy workpiece, excessive penetration of the molten aluminum alloy weld pool can cause pitting and wear on the electrode when extended weld times are used.